The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark-ignition engines and compression-ignition engines. Conventional spark-ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Conventional compression-ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both conventional gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies have been introduced for internal combustion engines. One such engine system comprises an internal combustion engine designed to operate in a controlled auto-ignition mode under specific engine operating conditions to achieve improved engine fuel efficiency, also referred to as homogeneous charge compression-ignition (HCCI) combustion mode. A spark-ignition system is employed to supplement the auto-ignition combustion process during specific operating conditions.
A typical HCCI engine operates in either the controlled auto-ignition combustion mode or the spark-ignition mode depending upon the engine speed and load. The HCCI combustion mode comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. Ignition of a cylinder charge is caused by compression of the cylinder charge under specific engine operating conditions. In the typical engine operating in HCCI combustion mode, the cylinder charge is nearly homogeneous in composition, temperature, and residual level at intake valve closing time. The typical engine operating in the HCCI combustion mode can further operate using stratified charge fuel injection to control and modify the combustion process, including using stratified charge combustion to trigger the HCCI combustion. Because auto-ignition is a distributed kinetically-controlled combustion process, the engine operates at a very dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low NOx emissions. The fuel/air mixture for auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this very dilute fuel/air mixture, an engine operating in the auto-ignition combustion mode can operate unthrottled to achieve diesel-like fuel economy. Furthermore, the HCCI engine can operate at stoichiometry with substantial amounts of EGR to achieve effective combustion. At medium engine speeds and loads, a combination of engine valve profile and timing (e.g., exhaust recompression and exhaust re-breathing) and fueling strategy has been found to be effective in providing adequate thermal energy to the cylinder charge so that auto-ignition during the compression stroke leads to stable combustion with low noise. One of the main issues in effectively operating an engine in the auto-ignition combustion mode has been to control the combustion process properly so that robust and stable combustion resulting in low emissions, optimal heat release rate, and low noise is achievable over a range of operating conditions. The benefits of auto-ignition combustion have been known for many years.
There is no direct control of start of combustion for an engine operating in the auto-ignition mode, as the chemical kinetics of the cylinder charge determine the start and course of the combustion. At engine operating conditions above certain limits, the HCCI engine transitions to spark-ignition combustion, at stoichiometry, in order to achieve stable combustion, manage emissions, and meet an operator torque request. The typical HCCI engine transitions between HCCI combustion mode and spark-ignition (SI) combustion mode, depending upon precalibrated and predetermined operating conditions. Often, the SI mode includes operating un-throttled at a stoichiometric air/fuel ratio.
The engine operating speed at which transition between HCCI and SI is possible is limited based upon engine hardware, especially the lift and duration of valve openings due to the camshaft profiles. By way of example, when a cam provides a four millimeter peak lift and an opening duration of 120 CA degrees, a transition between HCCI and SI operation has proven unachievable above an engine speed of about 2000 rpm.
It is advantageous to have a control system for a HCCI engine which effectively transitions between the HCCI combustion mode and the spark-ignition combustion mode. Furthermore, there is a need to effectively transition between SI and HCCI operation at higher engine operating speeds.
The invention described hereinafter comprises a method and a control scheme for operating the engine to control and maintain effective combustion during transitions between HCCI and SI combustion modes.